The present invention is directed to a method and apparatus for transmitting data between a central data station and a plurality of data terminals in a local area network.
The present method is directed to the transmission of binary coded data between a central data station and a plurality of data terminals in a local area network utilizing a network node connected to a means for input/output signal processing in the central data station, whereby the data transmission occurs with a standardized data transmission procedure wherein the address of one data terminal of the plurality of data terminals is contained in the transmitted data and every data terminal only transmits data after being called by the central data station, and whereby the data transmission also occurs with a given data transmission method from a group of base band transmission methods.
It is well know to establish data connection via network nodes in order to eliminate transmission lines and transmission equipment. For example, network nodes can be fashioned as interface multipliers or as concentrators. Via these network nodes, the terminal lines of an individual data terminal can be connected to a common connecting line that is connected to a central data station, for example a data processing system.
Interface multipliers are used in non-switchable network nodes wherein signals from a single interface at a first side are routed to a plurality of interfaces at a second side and the signals in the reverse direction are routed from the individual interfaces of the second side to the single interface at the first side. Concentrators are used in switchable network nodes and have controlled equipment for connection set-up and always produce only one data-transmissive path between a data terminal and the central data station. In both instances, data transmission equipment of different types can be connected at both sides of the network node, so that the form of transmission on the individual transmission sections is freely selectable. Previously known arrangements for transmitting data between a central data station and a plurality of data terminal equipment, particularly using a concentrator, use different forms for the data transmission on the different transmission sections. The concentrator therefore establishes mutual matching for the different data transmission forms. The matching, however, causes a considerable reduction in the data transmission rate. Moreover, the concentrators are extremely complex in structure and in terms of their control.
The form of data transmission is composed of a data transmission procedure and a data transmission method. The data transmission procedure specifies the method and the rules for the chronological sequence of data exchange between the communicating partners. The data transmission method indicates how different coding characters are physically transmitted on the lines, i.e., which pulse shapes and, potentially, which modulation is selected for the transmission of the data signals.
For example, one known data transmission procedure is the standardized data transmission procedure HDLC (high level data link control) that, for example, is described in Elektronik, 1980, No. 7, pages 64-69 and No. 8, pages 89-93. The HDLC data transmission procedure is distinguished by high data transmission reliability and can be easily used in combination with the above-cited, switchable network nodes and with data stations that are capable of utilizing the HDLC procedure.
For example, bipolar or AMI (alternate mark conversion) methods are two of many known data transmission methods, and belong to a group of base band transmission methods described in the book by P. Bocker, Datenuebertragung, Volume 1, pages 1-4 and 1-5, 1978, Springer-Verlag. In this data transmission method, the binary 0 of binary coded data has a transmission pulse 0 allocated to it and the binary 1 has a transmission pulse +A or -A allocated to it in alternation.
It is known that for data transmission the HDLC data transmission can be combined with the AMI data transmission method, but only in the context that either the individual data terminals are directly connected to the central data station or, if a network node is interposed, a different data transmission form is used after the network node. In the former instance, only relatively short distances between the central data station and the data terminal are feasible; whereas, in the second case, the aforementioned disadvantages exist when different data transmission forms are used on different transmission sections. When greater distances exist between the central data station and the data terminals, then modems are necessary, which means an additional outlay. At the same time, the line requirement is greatly increased since every terminal line is individually lengthened.